Simple Interest Calculator Australia 2026-27
Simple interest, and what it becomes when you compound it, add a monthly deposit, pay tax on it or hold it long enough for inflation to matter — with the balance drawn year by year.
Simple interest is principal times rate times time, and it is the one form of interest that never earns on itself: 10,000 at 6% earns 600 a year whether it is the first year or the tenth.
| Principal · Yr 0 | $10,000.00 |
|---|---|
| Principal · Yr 1 | $10,000.00 |
| Principal · Yr 2 | $10,000.00 |
| Principal · Yr 3 | $10,000.00 |
| Interest · Yr 0 | $0.00 |
| Interest · Yr 1 | $600.00 |
| Interest · Yr 2 | $1,200.00 |
| Interest · Yr 3 | $1,800.00 |
How the simple interest works
Simple interest is principal times rate times time, and it is the one form of interest that never earns on itself: 10,000 at 6% earns 600 a year whether it is the first year or the tenth. Choose a compounding frequency and the arithmetic changes completely — each period's interest joins the balance and earns in the next, so the same rate over the same term returns more the more often it compounds, and the gap widens with time rather than staying proportional. That difference is the whole reason a quoted rate means little without the compounding beside it. A regular deposit is added at the start of each period so it earns for that period, which is the convention a savings account uses. Tax comes off the interest, not the balance: interest is ordinary income in all three markets, taxed at your marginal rate rather than at a savings rate, and the after-tax line is what actually accrues to you. The real value discounts the final balance by inflation over the term — the number that answers whether the money grew or merely kept its place, and at a 6% return against 3% inflation it is roughly half of what the nominal figure suggests.
Formula: Simple: I = P × r × t · Compound: A = P(1 + r ÷ n)^(n × t) · real value = A ÷ (1 + i)^t
Australia Simple Interest Notes
Australian simple-interest examples often appear in short-term loans, informal repayment plans, school finance exercises, invoice interest, and basic savings comparisons.
Unlike compound interest, the interest is calculated only on the original principal, so the result is easier to explain but less realistic for most bank accounts.
Use this AUD version for a clear interest amount over days, months, or years when the agreement does not reinvest or compound interest.
For compounding savings, term deposits, or investment growth, compare this result with the compound-interest calculator.
Australian version note: this simple interest keeps the calculation anchored to AUD amounts, local product names, Australian tax language, and the way banks, employers, agencies, or advisers usually describe the inputs.
Local cues stay visible where they matter: ATO, PAYG, superannuation, Medicare levy, stamp duty, kilometres, comparison rate, APRA, Centrelink, GST, and Australian-dollar results are not rewritten into overseas vocabulary.
Use the output as an Australian estimate first, then sanity-check it against local quotes, lender criteria, government thresholds, state rules, or professional advice before relying on the number.
Simple interest = Principal × Rate × Time. Interest does not compound — it remains flat.
Simple interest by amount (5%, 3yr) Simple vs compound over time ($10k, 5%) Rate impact ($10k, 3yr) All calculations run 100% in your browser. The calculator code does not submit your figures to GlobalCalc to obtain a result. Understanding your resultSelect the question that matches where you are right now.
Your result shows the projected growth or return based on the rate, contribution, and time period you entered — using standard compound or simple interest formulas.
What to do with itUse this to set savings targets, compare investment options, or understand the impact of starting earlier. Adjust the rate and timeframe to model optimistic and conservative scenarios.
What it is notNot a guaranteed return. Actual investment outcomes depend on market conditions, fees, taxes, and timing that cannot be predicted with certainty.
AccuracyUses standard financial formulas with the inputs you provided. Calculator arithmetic runs in your browser; the calculator code does not submit your figures to GlobalCalc to obtain a result.
Savings and investment results are dominated by three factors: the rate of return, the time horizon, and regular contributions. Compounding amplifies all three over time.
Compound growthReturns on returns accelerate growth over time. The difference between 5% and 7% over 20 years is much larger than the 2% gap suggests — compounding is non-linear.
Regular contributionsAdding even small regular amounts dramatically increases the final balance. $100/week invested at 7% for 20 years grows to over $110,000 in contributions and $110,000+ in returns.
Time horizonStarting 5 years earlier often produces a larger final balance than doubling your contribution rate. Time is the most powerful variable in savings calculations.
To improve your savings outcome, focus on starting earlier, increasing contributions, and minimising fees and tax drag on returns.
Start now, increase laterStarting with a small amount today and increasing over time beats waiting to start with a larger amount. Time in the market matters more than timing the market.
Minimise feesA 1% annual fee on a $100k balance costs $1,000/year and compounds against you. Compare fee structures across savings and investment products.
Use tax-advantaged accountsSuper, offset accounts, and tax-free thresholds reduce the drag of tax on your returns — letting more of the growth compound for you.
Savings decisions connect to investment, tax, and retirement planning. Use these calculators to model the broader picture.
Set a savings goalWork backwards from a target amount to see how much you need to save each month.
Savings goal → Check compound growthModel how an initial investment grows with regular contributions over different time periods.
Compound interest → Factor in inflationSee what your future balance is worth in today's dollars after adjusting for inflation.
Inflation calculator → Related calculators 📈Compound InterestCompare with compound 🎯Savings GoalSavings planning 📉Loan RepaymentLoan interest 💹InflationReal returns 💡ROI CalculatorInvestment return 📊Net WorthSavings picture How simple interest works How it worksHow simple interest is calculated
Simple interest formula
Simple Interest = Principal × Rate × Time (P × R × T). Interest is calculated only on the original principal — it never compounds. A $10,000 loan at 5% for 3 years: SI = $10,000 × 0.05 × 3 = $1,500.
| Principal | Rate | Years | Simple interest | Total repaid |
|---|---|---|---|---|
| $5,000 | 5% | 2 | $500 | $5,500 |
| $10,000 | 5% | 3 | $1,500 | $11,500 |
| $20,000 | 7% | 5 | $7,000 | $27,000 |
| $50,000 | 4% | 10 | $20,000 | $70,000 |
Simple interest examples — savings and loans
| Amount | Rate | Period | Simple interest | Monthly interest |
|---|---|---|---|---|
| $5,000 | 4.5% | 1 yr | $225 | $18.75 |
| $10,000 | 5% | 3 yrs | $1,500 | $41.67 |
| $25,000 | 6% | 5 yrs | $7,500 | $125.00 |
| $100,000 | 5% | 2 yrs | $10,000 | $416.67 |
Simple vs compound interest — the difference over time
Why compound interest grows faster
With compound interest, each period's interest is added to the principal and then earns interest itself. With simple interest, the same dollar amount is added each period.
| Time period | $10k at 5% simple | $10k at 5% compound (annual) |
|---|---|---|
| 1 year | $10,500 | $10,500 |
| 5 years | $12,500 | $12,763 |
| 10 years | $15,000 | $16,289 |
| 20 years | $20,000 | $26,533 |
| 30 years | $25,000 | $43,219 |
The longer the period, the greater the advantage of compound interest. At 30 years, compound interest produces 73% more wealth than simple interest at the same rate.
When it's usedWhere simple interest is commonly used
Practical applications
Simple interest is used for: short-term personal loans (some pay-day and personal lenders), car loans (some dealer finance), government bonds, term deposits that calculate interest on the initial deposit only, and some bridging loans.
When savings accounts use compound interest
Most Australian savings accounts compound monthly or daily — meaning your interest earns interest. Even if the rate appears the same as a simple interest calculation, the compounding effect means you actually earn slightly more. Always check whether a rate is simple or compound (effective annual rate).
Worked examples Australian scenariosAustralian simple interest worked examples
| Principal | Rate | Time | Simple interest | Final amount |
|---|---|---|---|---|
| $5,000 | 5% p.a. | 2 years | $500 | $5,500 |
| $10,000 | 6% p.a. | 3 years | $1,800 | $11,800 |
| $20,000 | 4.5% p.a. | 5 years | $4,500 | $24,500 |
| $50,000 | 5% p.a. | 1 year | $2,500 | $52,500 |
| $2,000 | 9.99% p.a. | 6 months | $99.90 | $2,099.90 |
| $30,000 | 7.5% p.a. | 4 years | $9,000 | $39,000 |
| $1,000 | 12% p.a. | 90 days | $29.59 | $1,029.59 |
For partial-year calculations, multiply the rate by days/365 (or days/366 in a leap year). Most Australian short-term lenders use a 365-day basis even in leap years; banks typically use actual/365 for retail products.
Simple vs compound Side-by-sideSimple vs compound interest comparison over time
| Years | Simple ($10k @ 6%) | Compound monthly ($10k @ 6%) | Difference |
|---|---|---|---|
| 1 | $10,600 | $10,617 | $17 |
| 2 | $11,200 | $11,272 | $72 |
| 5 | $13,000 | $13,488 | $488 |
| 10 | $16,000 | $18,194 | $2,194 |
| 20 | $22,000 | $33,102 | $11,102 |
| 30 | $28,000 | $60,226 | $32,226 |
For short periods the difference is modest, but compound interest dominates over decades. For a 30-year retirement saving horizon, compound interest produces more than twice the final balance compared to simple interest at the same rate. Use our compound interest calculator to model the long-term growth.
Where it's used Real-world contextsWhere simple interest appears in Australian finance
Short-term loans and SACCs
Small Amount Credit Contracts (SACCs, often called payday loans) in Australia are capped under ASIC's national consumer credit regime. The maximum is a 20% establishment fee plus 4% monthly fee — typically calculated on a simple interest basis on the original principal, not the declining balance.
Bonds and fixed-income securities
Australian Government Bonds (AGBs) and most corporate bonds pay simple-interest coupons calculated on the face value of the bond, not on accrued interest. Coupon payments are typically semi-annual.
Some car finance and dealer loans
A minority of dealer-finance contracts use "Rule of 78" or pre-computed interest where interest is added upfront and the loan is repaid in equal instalments. This is mathematically similar to simple interest if held to term but penalises early repayment.
Overdue invoices and late fees
Statutory and contractual late-payment interest is almost always simple — typically 8% above the RBA cash rate per annum, applied on a daily basis to the overdue amount. Used widely in B2B contracts and statutory demand processes.
Educational examples and HSC maths
Simple interest is the foundation of HSC Mathematics Standard 2 and many TAFE financial-services modules. It's also the entry point for explaining time value of money before introducing compounding.
Day-count basis Day-count conventionsDay-count conventions used in Australian simple interest calculations
Actual/365 (the Australian retail default)
Most Australian retail banks and consumer credit products use Actual/365 — the actual number of days in the period divided by 365, regardless of whether the year is a leap year. A 90-day calculation uses 90/365.
Actual/Actual (government bonds)
Australian Government Bonds and some institutional securities use Actual/Actual — actual days divided by the actual length of the year (365 or 366). This is more accurate over leap years.
30/360 (some commercial contracts)
30/360 assumes every month has exactly 30 days, giving 360 days in a year. Common in some commercial loan documents inherited from US practice but rare in Australian retail products.
Quoting interest with day-count basis
When comparing two rates, always check the day-count basis. A rate of 5.00% on a 30/360 basis is equivalent to about 5.07% on Actual/365 — a small but material difference for large amounts. ASIC requires Australian credit contracts to state the day-count basis used.
Tax treatment ATO rulesTax treatment of simple interest earned and paid in Australia
Interest income
Simple interest earned on bonds, term deposits, or peer-to-peer lending is fully assessable income at your marginal rate. You should receive a tax statement from the issuer or platform each financial year. ATO pre-fill reports interest from major banks automatically.
Interest expense (for investors)
Interest paid on a loan used to acquire income-producing assets (e.g. a margin loan on shares, an investment property loan) is generally deductible. Simple interest on personal loans is not deductible if the loan was used for personal purposes.
PAYG withholding
For interest payments over $10 made by a business to a payee without a TFN/ABN, the payer must withhold 47% PAYG and remit to the ATO. The payee can recover the withholding via their tax return.
Reporting overdue invoice interest
Late-payment interest charged on overdue invoices is GST-free but is assessable income to the recipient. The income tax calculator models the impact of additional interest income on your marginal rate.
FAQFrequently asked Frequently asked questions about Australian simple interest
Where these figures come from
Savings and interest figures on this page are drawn from the Reserve Bank of Australia (cash rate and published deposit averages), APRA (the deposit-taker regulator), and ASIC MoneySmart (consumer guidance).
- RBA cash rate — RBA — Cash Rate.
- Deposit interest-rate data — RBA — Retail Deposit and Investment Rates (F4).
- Financial Claims Scheme (deposit guarantee up to $250k) — APRA — Financial Claims Scheme.
- Compound interest & savings strategy — ASIC MoneySmart — Saving.
- Inflation & CPI — ABS — Consumer Price Index.
Last checked: July 2026. Rates and thresholds are reviewed against the source of record each November, when annual adjustments for the following tax year are published.
You might also be interested in 📈Compound InterestCompare with compound 🎯Savings GoalSavings planning 📉Loan RepaymentLoan interest total 💹InflationReal return after CPI 💡ROIInvestment efficiency 📊Net WorthFull financial picture Rate this calculatorQuick answers
$10,000 at 5% simple interest — 3 years $1,500 interest
$10,000 at 5% simple interest for 3 years earns $1,500 in interest ($500/year). Total amount = $11,500. Formula: SI = P × R × T = $10,000 × 0.05 × 3.
| Principal | $10,000 |
|---|---|
| Rate | 5% p.a. |
| Time | 3 years |
| Simple interest | $1,500 |
| Total amount | $11,500 |
| vs compound (5%, 3yr) | $1,577 compound (more) |
- At 7% instead $2,100 interest (vs $1,500)
- Over 5yr instead $2,500 interest
- vs compound over 10yr $5,000 simple vs $6,289 compound
- When banks use this? Short-term loans, hire purchase
- Advantage for early repayment No compounding = lower total if repaid early
Simple vs compound — $10k at 5%, 10yr $5,000 vs $6,289
Over 10 years, $10,000 at 5% earns $5,000 with simple interest vs $6,289 with compound interest — a $1,289 difference. At 30 years, the gap grows to $18,219.
| Principal | $10,000 |
|---|---|
| Rate | 5% p.a. |
| Period | 10 years |
| Simple interest total | $5,000 |
| Compound interest total | $6,289 |
| Compound advantage (10yr) | $1,289 more |
- At 20 years $10k vs $16,533 (compound)
- At 30 years $15k vs $33,219 (compound)
- Practical implication Most savings = compound
- When does it matter? For short loans, simple is cheaper
- Compound daily vs annually Small difference on short periods
Simple interest formula — P × R × T Principal × Rate × Time
Simple interest is calculated as: SI = P × R × T, where P is the principal, R is the annual rate (as a decimal), and T is the time in years. The interest amount is the same every period — it does not compound.
| Formula | SI = P × R × T |
|---|---|
| P = Principal | Initial amount |
| R = Rate | Annual rate as decimal (5% = 0.05) |
| T = Time | In years |
| Example | $10k × 0.05 × 3 = $1,500 |
| Total amount | A = P + SI = $10k + $1,500 = $11,500 |
- vs compound interest SI is fixed; compound grows
- Monthly equivalent Annual SI ÷ 12
- Daily equivalent Annual SI ÷ 365
- When is SI used? Short-term loans, hire purchase, bills
- Simple vs compound (10yr) $5k vs $6,289 at 5%
When is simple interest used — practical examples Short-term loans and hire purchase
Simple interest is used in: short-term personal loans, some car loan products, hire purchase agreements, government bonds, and short-term bridging finance. Most long-term savings accounts and mortgages use compound interest.
| Short-term personal loans | Often simple interest |
|---|---|
| Some car loans | Dealer finance sometimes SI |
| Hire purchase | Retail finance often SI |
| Government bonds | Coupon payments are SI |
| Mortgages? | Compound — interest compounds monthly |
| Credit cards? | Compound — interest compounds daily |
- SI advantage for borrowers? Early repayment saves money vs compound
- SI savings products rare? Most savings accounts compound
- Check your loan type Product disclosure statement
- SI loans and extra repayments Extra payment reduces principal directly
- Hire purchase early exit Calculate remaining SI to clear
$5,000 at 6% — 2 years (simple interest) $600 interest, $5,600 total
$5,000 at 6% simple interest for 2 years earns $600 in interest: $5,000 × 0.06 × 2 = $600. Total amount = $5,600. With compound interest the total would be $5,618.
| Principal | $5,000 |
|---|---|
| Rate | 6% p.a. |
| Term | 2 years |
| Interest | $5,000 × 0.06 × 2 = $600 |
| Total amount | $5,600 |
| vs compound | Only $18 less — short-term minimal difference |
- When does compound beat simple? Longer terms and higher rates
- Bank savings: compound or simple? Bank savings accounts: compound daily
- ATO simple interest example Used in penalty interest calculations
- Hire purchase often uses SI Check your product disclosure
- Short term (<2yr): nearly identical Negligible difference
$10,000 at 5% — 3 years (simple interest) $1,500 interest, $11,500 total
$10,000 at 5% simple interest for 3 years earns $1,500 in interest. With compound interest, the total would be $11,576 — $76 more. The difference grows with time.
| Principal | $10,000 |
|---|---|
| Rate | 5% p.a. |
| Term | 3 years |
| Interest | $10,000 × 0.05 × 3 = $1,500 |
| Total amount | $11,500 |
| vs compound | $11,576 — $76 more with compounding |
- 3yr car loan often uses SI Fixed interest on finance
- Extra repayments on SI loan? Reduces principal — saves interest
- Compound alternative return $10k at 5% compound 3yr = $11,576
- Government bonds? Fixed coupon = simple interest structure
- Short personal loan check Read PDS for SI vs compound
$20,000 at 8% — 4 years (simple interest) $6,400 interest, $26,400 total
$20,000 at 8% simple interest for 4 years generates $6,400 in interest. With compound interest, the total would be $27,210 — $810 more. For loans, simple interest is cheaper for borrowers.
| Principal | $20,000 |
|---|---|
| Rate | 8% p.a. |
| Term | 4 years |
| Interest | $20,000 × 0.08 × 4 = $6,400 |
| Total amount | $26,400 |
| vs compound | $27,210 — $810 more with compounding |
- Car loan SI vs compound? SI loan: $6,400; Compound: $7,210 more expensive
- For savings: compound wins For loans: SI is cheaper for borrower
- Early repayment on SI loan? Saves remaining simple interest exactly
- Rate check 8% SI = 8% p.a. flat — advertised differently?
- Total interest as % of loan $6,400 ÷ $20,000 = 32% — significant
$50,000 at 7% — 5 years (simple interest) $17,500 interest, $67,500 total
$50,000 at 7% simple interest for 5 years earns $17,500 in interest. With compound interest, $70,128 would be earned — $2,628 more, showing how compounding accelerates over longer terms.
| Principal | $50,000 |
|---|---|
| Rate | 7% p.a. |
| Term | 5 years |
| Interest | $50,000 × 0.07 × 5 = $17,500 |
| Total amount | $67,500 |
| vs compound | $70,128 — $2,628 more |
- 5yr term: compound starting to diverge Gap grows each year after this
- For borrowing: SI is cheaper $17,500 vs $20,128 interest
- For lending/saving: compound is better $17,500 vs $20,128 earned
- Investment bond example Government bonds use coupon = SI
- At 10yr: gap much larger SI $35,000 vs compound $48,357
$100,000 at 6% — 10 years (simple interest) $60,000 interest, $160,000 total
$100,000 at 6% simple interest for 10 years earns $60,000 in interest. With compound interest, $79,085 would be earned — $19,085 more. This gap illustrates why investors prefer compound returns.
| Principal | $100,000 |
|---|---|
| Rate | 6% p.a. |
| Term | 10 years |
| Interest | $100,000 × 0.06 × 10 = $60,000 |
| Total amount | $160,000 |
| vs compound | $179,085 — $19,085 more |
- Rule of 72: SI vs compound SI takes 16.7yr to double; compound takes 12yr
- For mortgages? Mortgages use compound — more expensive for borrower
- Government bonds (10yr) ~$42,000–$45,000 coupon at current rates
- Long-term savings Always prefer compound structure for savings
- ATO interest on tax debts Uses SI typically — check ATO notices
$10,000 at 3% — 3 years (simple interest) $900 interest, $10,900 total
$10,000 at 3% simple interest for 3 years earns $900 in interest. At 3%, this barely covers inflation — real value is nearly unchanged over 3 years.
| Principal | $10,000 |
|---|---|
| Rate | 3% p.a. |
| Term | 3 years |
| Interest | $10,000 × 0.03 × 3 = $900 |
| Total amount | $10,900 |
| Real value after inflation (3%) | Purchasing power: unchanged |
- Inflation-adjusted? 3% inflation erases 3% return
- Savings account comparison? 5% compound significantly better
- For a loan at 3%? $900 cost = affordable personal loan
- Car finance check 3% advertised — but compound or SI?
- Use case Short-term loan repayment calculation
$10,000 at 5% — 3 years (simple interest) $1,500 interest, $11,500 total
$10,000 at 5% simple interest for 3 years earns $1,500 in interest. This is the typical return from a term deposit or fixed income product.
| Principal | $10,000 |
|---|---|
| Rate | 5% p.a. |
| Term | 3 years |
| Interest | $10,000 × 0.05 × 3 = $1,500 |
| Total amount | $11,500 |
| vs compound (5%, 3yr) | $11,576 — $76 more |
- Term deposit comparison 3yr term deposit at 5% — similar
- Monthly coupon option? Some bonds/TDs pay monthly interest
- Tax on $1,500? $285 at 19%, $488 at 32.5%
- vs savings account compound $11,576 vs $11,500 — negligible
- Suitable for? Predictable, low-risk 3yr goal
$10,000 at 7% — 3 years (simple interest) $2,100 interest, $12,100 total
$10,000 at 7% simple interest for 3 years earns $2,100 in interest. The compound version over 3 years earns $2,250 — $150 more. Still a strong real return above inflation.
| Principal | $10,000 |
|---|---|
| Rate | 7% p.a. |
| Term | 3 years |
| Interest | $10,000 × 0.07 × 3 = $2,100 |
| Total amount | $12,100 |
| vs compound (7%, 3yr) | $12,250 — $150 more |
- vs inflation (3% over 3yr) Real gain: ~$800 after inflation
- Car loan at 7% SI $2,100 interest on $10k — compare lenders
- vs 5% SI $1,500 vs $2,100 — 40% more income
- Compounding starts to matter? Gap grows beyond 3yr
- ASX comparison 7% SI vs 8-10% compound — shares win long term
$10,000 at 10% — 3 years (simple interest) $3,000 interest, $13,000 total
$10,000 at 10% simple interest for 3 years earns $3,000 in interest. At 10%, this is a high-return scenario — typically only seen in higher-risk investments or high-rate personal loans.
| Principal | $10,000 |
|---|---|
| Rate | 10% p.a. |
| Term | 3 years |
| Interest | $10,000 × 0.10 × 3 = $3,000 |
| Total amount | $13,000 |
| vs compound (10%, 3yr) | $13,310 — $310 more |
- 10% SI loan cost $3,000 on $10k — consider refinancing
- vs 7% alternative? Save $900 in interest over 3yr
- Investment at 10% realistic? ASX historical average — with compounding
- Short-term loan at 10% Not unreasonable if secured
- Check for compound? Read loan documents carefully
$10,000 at 15% — 2 years (simple interest) $3,000 interest, $13,000 total
$10,000 at 15% simple interest for 2 years earns $3,000 in interest. This rate is very high — typical of unsecured personal loans and some credit products. Urgently consider refinancing.
| Principal | $10,000 |
|---|---|
| Rate | 15% p.a. |
| Term | 2 years |
| Interest | $10,000 × 0.15 × 2 = $3,000 |
| Total amount | $13,000 |
| vs refinance at 8%? | $1,600 interest — saves $1,400 |
- Refinance at 8%? Save $1,400 over 2yr
- Is 15% legal? Yes — legal but expensive
- Credit card comparison 20% compound card is worse
- Improve credit score Opens access to lower rates
- Check comparison rate 15% advertised may be higher with fees
Comparing compound vs simple: 5yr difference at 7% $19,672 vs $17,500 on $10,000
At 7% over 5 years on $10,000: simple interest earns $3,500 ($13,500 total); compound interest earns $4,672 ($14,672 total). The compound advantage is $1,172 — and this gap accelerates every year.
| Period | 5 years |
|---|---|
| Rate | 7% p.a. |
| SI result | $3,500 interest = $13,500 total |
| Compound result | $4,672 interest = $14,672 total |
| Compound advantage | $1,172 more |
| Year 10 compound advantage | $9,672 vs $7,000 — $2,672 more |
- The gap accelerates Each year, compound earns on prior interest
- For savings: always choose compound Any savings product should be compound
- For loans: SI is cheaper borrower cost Simple interest loan is better for you
- Rule of 72 comparison SI doubles in 14.3yr; compound in 10.3yr
- Why super is compound 15% × compounding = powerful
Comparing compound vs simple: 10yr at 7% $19,672 vs $17,000 on $10,000
At 7% over 10 years on $10,000: simple interest = $17,000 total; compound = $19,672 total. Compound earns $2,672 more — and this is without additional contributions.
| Period | 10 years |
|---|---|
| SI total | $7,000 interest = $17,000 |
| Compound total | $9,672 interest = $19,672 |
| Compound advantage | $2,672 more |
| After 20yr compound advantage | $38,697 vs $24,000 — $14,697 more |
| After 30yr compound advantage | $76,123 vs $31,000 — $45,123 more |
- Time amplifies compound benefit The longer, the bigger the gap
- Savings: always compound structure Maximises return
- Investment returns: compound by nature ASX returns compound automatically
- Super fund returns Compound over 30yr — massive impact
- ASIC Moneysmart rate 7% compound used in official projections
Comparing compound vs simple: 20yr at 7% $38,697 vs $24,000 on $10,000
At 7% over 20 years on $10,000: simple interest = $24,000 total; compound = $38,697 total. Compound earns $14,697 more — 61% more than simple interest.
| Period | 20 years |
|---|---|
| SI total | $14,000 interest = $24,000 |
| Compound total | $28,697 interest = $38,697 |
| Compound advantage | $14,697 more (+61%) |
| Doubles in? | SI: never doubles; Compound: 10.3yr |
| Best for 20yr? | Compound investment — always |
- Super fund over 20yr $10k at 7.5% compound = $42,478
- Index ETF vs savings account Compound vs near-compound — similar structure
- Employer SG over 20yr $12k /yr at 7.5% = $590,000
- Long-term savings decision Always choose compound structure for savings
- The 20yr lesson Time in market beats almost everything
Comparing compound vs simple: 30yr at 7% $76,123 vs $31,000 on $10,000
At 7% over 30 years on $10,000: simple interest = $31,000 total; compound = $76,123 total. Compound earns $45,123 more — 3× the simple interest result.
| Period | 30 years |
|---|---|
| SI total | $21,000 interest = $31,000 |
| Compound total | $66,123 interest = $76,123 |
| Compound advantage | $45,123 more (+145%) |
| Rule of 72 at 7% | Doubles 3× in 30yr = $80,000 approx |
| Best use of 30yr? | Super, index ETFs — compound structure |
- Why start early? Every extra year of compound adds disproportionately
- $10k at age 25 → 65 (40yr) $149,745 compound vs $38,000 SI
- Superannuation parallel SG on $50k salary: $6k /yr × 30yr × 7.5% = $671k
- Simple vs compound summary 30yr: compound earns 145% more
- Reinvest all returns Key to achieving compound result
Worked examples
| Inputs | Total Amount | Note |
|---|---|---|
| 10,000 at 6% for three years, simple | $11,800 | 1,800 of interest — 600 a year, every year |
| The same, compounded monthly | $11,967 | the same rate and term returns more, because each month earns on the last |
| With 200 a month added | $19,873 | the deposits do most of the work over a short term |
FAQFrequently asked questions
What is the simple interest formula?
Simple Interest = Principal × Rate × Time (P × R × T). Where rate is expressed as a decimal (5% = 0.05) and time is in years. Total amount = Principal + Simple Interest.
What is the difference between simple and compound interest?
Simple interest is calculated only on the original principal — the interest amount is the same each period. Compound interest adds each period's interest to the principal, so subsequent interest is calculated on a growing balance. Over time, compound interest produces significantly more growth.
Do Australian banks use simple or compound interest?
Most Australian savings accounts and term deposits use compound interest (calculated daily or monthly). Most personal loans use compound interest charged monthly. Some short-term loan products and bonds use simple interest. Always check the product disclosure statement.
When is simple interest better for the borrower?
Simple interest loans can be cheaper than compound interest loans at the same rate if the loan is repaid early — because interest does not accumulate on unpaid interest. This is most relevant for short-term loans and early repayment scenarios.
How do I calculate simple interest for a partial year?
Convert the time to a fraction of a year. For 90 days at 5%: T = 90/365 = 0.2466. SI = $10,000 × 0.05 × 0.2466 = $123.29. Most Australian retail products use the actual/365 basis even in leap years.
Is simple interest taxable in Australia?
Yes — interest income is assessable at your marginal tax rate regardless of whether it's calculated on a simple or compound basis. Issuers will provide an annual interest summary at tax time; ATO pre-fill imports most major-bank interest data automatically.
Can I claim simple interest as a tax deduction?
Interest is deductible only if the underlying loan was used to acquire an income-producing asset (e.g. investment property, share portfolio, business inventory). Interest on personal-purpose loans (holidays, weddings, primary residence) is not deductible.
What is the "Rule of 78" and how does it relate to simple interest?
Rule of 78 is a pre-computation method where total interest for the loan term is calculated upfront and allocated to instalments in a front-loaded pattern. Early repayment under Rule of 78 returns less interest than equivalent declining-balance interest. Rare in Australian consumer credit since the National Credit Code came into force.
Do Australian Government Bonds use simple interest?
AGB coupon payments are simple-interest-style — calculated on the face value of the bond, paid semi-annually. The yield-to-maturity calculation that determines a bond's traded price does account for compounding of reinvested coupons, but the cashflows themselves are simple.
Why is simple interest used in HSC maths textbooks?
Simple interest is the foundational concept of time value of money. Once students understand P × R × T, they can move to compound interest, present/future value, and amortisation. HSC Mathematics Standard 2 and TAFE financial-services modules both start here.
Can simple interest be negative?
Yes — in special cases like the RBA's near-zero rate experiments and some negative-yielding bonds during 2020-21. A negative simple interest rate produces a negative return: you receive less back than you deposited. Rare but possible.
How is simple interest applied to overdue invoices?
Under the Late Payments Act regime and most B2B contracts, late-payment interest accrues at a stated rate (often 8% above RBA cash) on a simple, daily-rate basis from the date payment was due until the date paid. The interest itself is GST-free and assessable income to the recipient. Sources
The sources behind this calculator
- CFA Institute — Quantitative Methods: The Time Value of Money — the discounting, annuity and IRR conventions used here
- US Federal Reserve — Regulation Z (Truth in Lending), APR calculation — why APR includes fees where a nominal rate does not
- ASIC MoneySmart — the Australian regulator's own consumer calculators
Last checked: September 2026. These are the standard textbook formulas; the conventions named are those of the CFA Institute curriculum and ISO 80000 usage.